Composite structures have become a practical solution to developing materials with specialized properties for specific applications. Metal matrix composites have become especially useful in specific aeronautical applications. Composite materials combine features of at least two different materials to arrive at a material with desired properties. For purposes of this specification, a composite is defined as a material made of two or more components having at least one characteristic reflective of each component. A composite is distinguished from a dispersion strengthened material in that a composite has particles in the form of an aggregate structure with grains, whereas, a dispersion has fine particles distributed within a grain. Dispersoids strengthen a metal by increasing the force necessary to move a dislocation around or through dispersoids. Experimental testing of dispersion strengthened metals has resulted in a number of models for explaining the strength mechanism of dispersion strengthened metals. The stress required of the Orowan mechanism wherein dislocations bow around dispersoids leaving a dislocation loop surrounding the particle is given by: ##EQU1## where .sigma..sub.or is the stress of a dislocation to bow around a dislocation with the Orowan mechanism, G is the shear modulus, b is the Burgers vector, M is the Taylor factor and L is the interdispersoid distance. The appropriate interdispersoid distance is the mean square lattice spacing which is calculated by the following equation: EQU L=[(.pi./f).sup.0.5 -2](2/3).sup.0.5 r
where f is the volume fraction of dispersoid and r is the dispersoid radius. Dispersoids with an interparticle distance of much more than 100 nm will not significantly increase yield strength. Optimum dispersion strengthening is achieved with, for example, 0.002-0.10 volume fraction dispersoids having a diameter between 10 and 50 nm. Decreasing interdispersoid spacing is a more effective means of increasing dispersion strengthening than increasing volume fraction because of the square root dependence of volume fraction in the above equation.
A major factor in producing metal matrix composites is compatibility between dispersion strengtheners and the metal matrix. Poor bonding between the matrix and the strengtheners significantly diminishes composite properties. A composite structure has properties that are a compromise between the properties of two or more different materials. Room temperature ductility generally decreases proportionally and stiffness increases proportionally with increased volume fraction of particle stiffener (hard phase) within a metal matrix. Conventional aluminum SiC composites have been developed as high modulus lightweight materials, but these composites typically do not exhibit useful strength or creep resistance at temperatures above about 200.degree. C.
A mechanically alloyed composite of aluminum matrix with SiC particles is disclosed in U.S. Pat. No. 4,623,388. However, these alloys lose properties at elevated temperatures.
A high modulus mechanically alloyed aluminum-base alloy is disclosed in U.S. Pat. No. 4,834,810. The aluminum matrix of this invention is strengthened with Al.sub.3 Ti intermetallic phase, Al.sub.2 O.sub.3 and Al.sub.4 C.sub.3 formed from stearic acid and/or graphite process control agents. The fine particle dispersion strengthening mechanism of the '810 patent produced an alloy having high modulus and relatively high temperature performance.
It is an object of this invention to produce an aluminum-base metal matrix composite having sufficient bonding between the metal matrix and particle stiffeners.
It is another object of this invention to produce a mechanically alloyed aluminum-base alloy having increased retained ductility upon addition of stiffener particles.
It is another object of this invention to produce a lightweight aluminum-base alloy having practical engineering properties at higher temperatures.